Nannostomus minimus (from the Greek: nanos = small, and the Latin stoma = relating to the mouth; from the Latin: minimus = of the smallest size), commonly known as the least pencilfish, is a freshwater species of fish belonging to the characin family Lebiasinidae. It is restricted to Guyana, where it has been reported from the Potaro and Mazaruni rivers.
N. minimus reaches a length of . When described in 1909, it was the smallest of the known pencilfishes; that distinction has since passed to N. anduzei.